1. Field of the Invention
The present invention relates in general to a hydraulic shock-absorber for a vehicle suspension, in particular a twin-tube hydraulic shock-absorber. More specifically, the present invention relates to a hydraulic shock-absorber comprising a hydraulic stop member arranged to operate during the compression stroke of the shock-absorber.
2. Description of the Related Art
A hydraulic shock-absorber as specified above is known from CN 103 953 676 A.
As it is known, a twin-tube hydraulic shock-absorber for a vehicle suspension typically comprises an outer cylindrical tube, an inner cylindrical tube coaxial with the outer cylindrical tube and defining with the latter an annular chamber which is filled with gas in an upper portion thereof, a rod arranged coaxially with the two cylindrical tubes and partially projecting from the latter, and a piston which is slidably mounted in the inner cylindrical tube and is attached to the bottom end of the rod. The piston separates the inner volume of the inner cylindrical tube into a rebound chamber and a compression chamber, in which a damping fluid (oil) is contained. The piston is provided with a first pair of check valves, namely a compensation valve which controls the flow of the damping fluid from the compression chamber to the rebound chamber during the compression stroke of the shock-absorber and a rebound valve which controls the flow of the damping fluid from the rebound chamber to the compression chamber during the rebound stroke of the shock-absorber. A valve assembly is arranged on the bottom of the shock-absorber and comprises a second pair of check valves, namely a compression valve which controls the flow of the damping fluid from the compression chamber to the annular chamber during the compression stroke and an intake valve which controls the flow of the damping fluid from the annular chamber to the compression chamber during the rebound stroke.
Traditionally, a hydraulic shock-absorber for a vehicle suspension is provided with a first end stop member, which is arranged inside the shock-absorber and is configured to operate during the rebound stroke of the shock-absorber, and a second end stop member, which is arranged outside the shock-absorber and is configured to operate during the compression stroke of the shock-absorber.
The end stop members may be of elastic or hydraulic type.
The elastic end stop members are made of high-stiffness elastic material (for example polyurethane) and have the function of ensuring that when shock-absorber reaches the end-of-travel position, either during the rebound stroke or during the compression stroke, there are elastic shocks between the surfaces that come into contact, instead of shocks between metal surfaces. The elastic end stop members behave therefore as springs which at the end-of-travel positions act in parallel with the mechanical springs (main spring) of the suspension and have a much higher stiffness than that of the mechanical spring of the suspension. However, the elastic end stop members do not operate in any intermediate point of the stroke of the shock-absorber.
The elastic end stop members have, among others, the following disadvantage. In case of a heavy shock on the stop members, either during the rebound stroke or during the compression stroke, caused for example by obstacles (holes or bumps) on the road, the kinetic energy of the suspension is converted into elastic deformation energy of the stop member in question. The elastic energy stored by the stop member is completely (or almost completely) released in the subsequent phase of change of direction of the motion. Accordingly, after the shock the suspension continues to oscillate without its motion being sufficiently dampened, and these oscillations adversely affect the adherence of the tyre with the road, and hence result in a worse road-holding of the vehicle.
In order to overcome the aforesaid disadvantage of the elastic stop members, it is known to use hydraulic stop members, either alone or in combination with elastic stop members. The hydraulic stop members are energy dissipating devices which allow, when an end-of-travel position is reached, to hydraulically dissipate the kinetic energy of the suspension by forcing a certain amount of oil contained in the shock-absorber to laminarly flow through suitably calibrated passages. In this way, the energy produced as a result of a shock is hydraulically dissipated and is not returned to the suspension in the subsequent phase of change of direction of the motion. The oscillations of the suspension are thus dampened, which increases the adherence of the tyre with the road and therefore improves the road-holding of the vehicle. If, on the other hand, the same performances in terms of road-holding are kept, it is possible, by virtue of the hydraulic stop members, to reduce the size of the tyres and hence their rolling resistance, thereby decreasing fuel consumption and emissions.
Furthermore, since the hydraulic stop members act in parallel with the respective elastic stop members, part of the energy generated by the shock is hydraulically absorbed and it is therefore possible to reduce the size of the elastic stop members and of the relating supports.
Finally, while the action of the elastic stop members depends on the deformation to which these members are subject, the action of the hydraulic stop members is proportional to the speed with which the rod of the shock-absorber reaches the end-of-travel position. The hydraulic stop members allow therefore to manage in a much more progressive manner the impacts occurring when the end-of-travel positions of the shock-absorber are reached.
The hydraulic stop members are almost exclusively used as rebound stop members. Documents EP 2 302 252, WO 2014/165951, US 2014/0360353 and WO 2005/106282 disclose examples of hydraulic stop members which act during the rebound stroke of the hydraulic shock-absorber and are therefore intended to be mounted in the rebound chamber of the shock-absorber.
There are however some examples of hydraulic stop members acting during the compression stroke.
EP 1 717 478 discloses a hydraulic stop member for a motor-vehicle comprising a cup-shaped body, which is adapted to be fixedly mounted inside the compression chamber of the shock-absorber, and a plunger, which is adapted to be attached to the bottom end of the rod of the shock-absorber (i.e. the end facing the bottom of the shock-absorber). The plunger is able to slide along a cylindrical side wall of the cup-shaped body. This wall has a number of holes to allow the oil to flow out of the cup-shaped body when the plunger is urged towards the latter, during the compression stroke of the shock-absorber, and hence compress the oil contained in a working chamber of the cup-shaped body. The plunger comprises a shaft and a head, the head being slidably mounted between a shoulder formed by the shaft and a nut mounted at the free end of the shaft. Conduits are provided along the perimeter of a central hole of the head (hole through which the shaft extends) to allow the oil to flow therethrough when the head is in abutment against the nut during the rebound stroke of the shock-absorber.
In such a stop member the motion of the plunger, and hence the motion of the rod of the shock-absorber, is dampened during the compression stroke of the shock-absorber as a result of the oil flowing out of the cup-shaped body through the holes provided in the cylindrical side wall of that body. As the plunger moves towards the inside of the cup-shaped body, an increasing number of holes is closed by the plunger, whereby the damping effect produced by the device is increased. The change in damping that can be obtained with such a stop member is thus a discrete change, not a continuous one.